Add complex filters for RunMetaEntries

[meksor]

Adds a filter class for the RunMetaEntry model.
Enabling filters like:

backend.meta.tabulate(key__in=[...], run={"default_only":False})

Full current filterset from the OpenAPI spec:

{
  "id": 0,
  "type": "string",
  "run_id": 0,
  "key": "string",
  "value_int": 0,
  "value_str": "string",
  "value_float": 0,
  "value_bool": true,
  "id__in": [
    0
  ],
  "type__in": [
    "string"
  ],
  "type__like": "string",
  "type__ilike": "string",
  "type__notlike": "string",
  "type__notilike": "string",
  "run_id__in": [
    0
  ],
  "run_id__gt": 0,
  "run_id__lt": 0,
  "run_id__gte": 0,
  "run_id__lte": 0,
  "key__in": [
    "string"
  ],
  "key__like": "string",
  "key__ilike": "string",
  "key__notlike": "string",
  "key__notilike": "string",
  "value_int__in": [
    0
  ],
  "value_int__gt": 0,
  "value_int__lt": 0,
  "value_int__gte": 0,
  "value_int__lte": 0,
  "value_str__in": [
    "string"
  ],
  "value_str__like": "string",
  "value_str__ilike": "string",
  "value_str__notlike": "string",
  "value_str__notilike": "string",
  "value_float__in": [
    0
  ],
  "value_float__gt": 0,
  "value_float__lt": 0,
  "value_float__gte": 0,
  "value_float__lte": 0,
  "run": {
    "default_only": true,
    "is_default": true
  }
}

[glatterf42]

Thanks for this template. The docs failure is my fault, I deleted the branch of my fork after sphinx-contrib/openapi merged my desired changes without first updating our dependency to their official version again. #27 is going to fix that, I'll then rebase this PR.

[glatterf42]

@danielhuppmann Is this complete and the behaviour as you envisioned?

[glatterf42]

According to my local pytest run (pytest tests --cov=ixmp4), the new tests don't add a single line to the coverage. I'm not sure if I'm missing something here or if they are still useful.

[pmussak]

According to my local pytest run (pytest tests --cov=ixmp4), the new tests don't add a single line to the coverage. I'm not sure if I'm missing something here or if they are still useful.

I reviewed the tests and I consider them useful. Although I am not familiar with the strategy for writing tests within this project.

I added tests to cover the filtering by meta-key.